Even if he is fairly old and not insanely prolific, he should not be sold below 30mil. His work ethic, mild injury record, pedigree and goalscoring ability in every league and competition should guarantee that price. 100% sure a team like West Ham would be willing to pay that.
But if he has a fairly regular position in the squad in the future, then I wouldn't mind him staying for 1-2 more seasons and leaving on a free. Since the departures and ageing of some our stars we will soon lack leaders and especially ones with work ethic like Mandzu. Not to mention he is still actually a pretty damn luxury to have as a "backup" target man.